Raman scattering was discovered by C. V. Raman in 1928. Raman received the Nobel Prize in Physics in 1930 for his work on the scattering of lights and the discovery of the effect named after him.
Raman scattering is an inelastic scattering of a photon which creates or annihilates an optical phonon. In simpler words, Raman scattering is the result of the interaction of incident light (photons) with chemical molecular vibrations (phonons). A unique chemical molecular structure results in a unique Raman scattering spectrum (that's why it is also called Raman fingerprint spectroscopy). Therefore, Raman scattering provides spectral fingerprint details about the chemicals, and can also be used to distinguish molecular isomers or even chiral molecules from each other.
Raman spectroscopy was commercially available after invention of lasers in late 1960. In a standard setup, a laser (from UV to near infrared) is used to illuminate the testing chemicals in solid, liquid or gas forms. The reason to use a laser is that only single-wavelength photons will interact with the chemicals to eliminate the overlaps of scattering peaks from photons (lights) with various wavelengths. That is the reason why it took 30 years before Raman spectroscopy got into the real applications after its discovery. Once the scattering lights, after dispersion, are collected by a photon detector such as Charge-Coupled Devices (CCD) or CMOS detector, a Raman spectrum is collected. The Raman shift is defined as the wavelength spacing between the scattering light wavelength and incident light wavelength (laser wavelength). The positions of the peaks correspond to the vibration strengths of various molecular bonds, thus provide a spectral fingerprint of the molecules.
Raman scattering finds wide range of applications in many application areas: pharmaceutical, chemical, biological, medical, life science, materials science, semiconductors, art restoration, food safety, environmental, forensic, homeland security, and so on.
Although Raman scattering is an extremely useful analytical tool, it suffers one major draw-back: the scattering signal is very weak due to the very small scattering cross section of molecules. Typically, only about 10−8 of the incident photons on the chemicals will undergo Raman scattering. Of course, high power laser and high sensitivity CCD detector can be used to improve the scattering signals but coming with the extra costs, additional hardware, and unexpected sample damage. Because of the weak scattering signals, normal Raman scattering application is relatively broad but still very limited.
Surface-enhancement effect by using a roughed surface was found to boost Raman scattering signal. The so-called Surface-Enhanced Raman Spectroscopy (SERS) was developed (M. Fleischmann, P. J. Hendra, and A. J. McQulillan, “Raman Spectra of Pyridine Adsorbed at a Silver Electrode”, Chem. Phys. Lett., 26, 123 (1974)). The surface can be formed by deposition of metallic particles or clusters. In many cases, nano-particles of silver or gold are made in solutions, and a flat substrate such as glass is used to collect the nano-particles. Then the surface is either immersed in solution to be measured, or the solution is spread on the surface. The laser beam is directly illuminated on the surface with the nano-particles, and scattering lights are collected by a detector. With the interaction between the nano-particles and measured chemicals, an enhanced Raman spectrum is obtained. The Raman signal could be enhanced by up to 9-10 orders (or even higher) of magnitude as comparing to the normal Raman scattering.
Zhongfan Liu and his colleagues (Nanotechnology, 15, 357 (2004)) demonstrated that Raman signal enhancement gets stronger as the average particle distance (spacing) decreased below 100 nm. More importantly, the significant enhancement takes off when the particle distance is close to or almost equal to the particle diameter. Furthermore, the enhancement is even stronger as the particle diameter gets smaller than 100 nm. In summary, Raman scattering will be greatly enhanced after interaction with nano-particle surfaces, especially with particles with sizes less than 50-100 nm.
The surface-enhanced Raman scattering phenomena can be explained by interaction between photons (laser) with localized electromagnetic field enhancement and chemical enhancement (see discussions by A. M. Michaels, et. al. J. Am. Chem. Soc. 121, 9932-39 (1999)).
Many research groups around the world demonstrated SERS. The enhancement can be repeated from one lab to another. One of the teams working on SERS in the last few years is from Intel (J. P. Roberts, Biophotonics International, Dec. 22, 2003). The Intel team used a porous silicon structure with coatings of noble metals such as silver on the surface. Intel demonstrated that the enhancement increases as the porous silicon pore-size decreases. All the experiments including the work from Intel can be repeated by another team, but it is difficult to reproducibly demonstrate the same level of enhancement.
Accordingly, there is a need to develop well-controlled nano-surface structures at low cost in order to realize commercialization of SERS for various applications. OptoTrace [U.S. patent application Ser. No. 10/852,787] disclosed the production of nano-surface structures, typically regular arrays of rods or holes with dimensions as small as 5 nm, without using the expensive lithographic method to define features. The work demonstrated the solutions for resolving reproducibility issues of SERS devices.
However, there are increasing demands for further improving SERS detection sensitivity for applications ranging from cargo inspection, food inspection, environment monitoring, disease diagnosis, to forensic and homeland security. Therefore there is a need to improve the performance of SERS devices and processing techniques for making the same.